1. Field of the Invention
The present invention pertains to filter systems. More particularly, the present invention relates to screen packs of the type used in extrusion processes, for example.
2. Description of Prior Art
In an extrusion process, material is forced through a forming die, after which it is cooled or chemically hardened. Different types of machines are used to drive the material to be extruded, such as one or more rotating screws of variable pitch. The driving mechanism may be capable of applying considerable pressure to force the extrusion material through the die.
In particular, in the extrusion of polyolefins and other types of polymers, for example, the extrusion material is heated as it is being compressed by the driving mechanism, and different additives are introduced into the material as well. The final product is then forced through one or another type of die or other mechanical device to form various finished products, such as strands, pellets or film. After all of the additives are introduced into the extrusion material, and the material is heated and compressed, some of the additive material may remain in its original, unreduced state in which it was introduced into the extrusion system. Also, some of the contents of the system may have solidified during the compression and heating of the material. Finally, some impurities may be in the system, and must be removed before the finished product is formed. Consequently, a filter system is incorporated in the extrusion system, generally in a tubular section just prior to the extruding material entering the final section of the extrusion line where the extrusion die is located. The filter system screens out the unwanted impurities and unprocessed additives, and catches any plastic particles that may have solidified, ensuring that only the fluid portion of the polymer is presented to the extruder die. In most extruding machines the polymer extrusion material passes through a circular, cylindrical portal when leaving the screw section to pass through the filtration system and on to the die section. Consequently, most filtration systems are flat, disc shaped, wherein the polymer must pass through a circular, flat filter oriented perpendicularly to the longitudinal axis of the extrusion line and, therefore, to the general direction of the material flow. The flat filter system includes a mesh held in place by a back, or support. The filter support is a flat metal plate with holes passing through it; the mesh, or filter screen may include multiple layers of screening material with holes smaller than the holes through the support, and wherein the size of the screen holes determine the smallest size particles which may pass through the filter system. The support is provided to anchor and strengthen the filter screen, and prevent it from being deformed or otherwise damaged by the polymer flow.
The total cross-sectional area available for polymer flow through a flat filter system is limited by how large the holes through the filter support may be while still providing sufficient support for the filter screen, and how close together the filter support holes may be while still allowing sufficient strength for the filter support to counter the force of the polymer flow. While a stronger filter support may be provided using a thicker-walled support, the thicker wall of the filter support makes the length of the flow passages through the filter support longer, thereby providing greater resistance to the polymer flow through the filter support passages. The cross-sectional area available for polymer flow through the filter system may be increased by deviating the filter system shape from a flat disc. One variation to provide greater cross-sectional flow area to the polymer flow is provided by a cone filter support in conjunction with a cone screen filter, with the vertex of the cone-shaped filter system pointing downstream in the polymer flow. Another variation utilized to obtain greater cross-sectional flow area for the polymer includes a flat base to which are attached a plurality of tubes positioned mutually parallel and oriented longitudinally relative to the tubular structure within which the filter system is positioned, that is, longitudinally relative to the flow of the polymer material along the extrusion line. Each of the filter support tubes has numerous holes therethrough, perpendicular to the longitudinal axis of the filter system, and a filter screen is positioned on each of the support tubes; a filter screen is positioned inside each support tube if the tubes are directed downstream; a filter screen is positioned on the outside of each support tube if the support tubes are oriented upstream.
Although the cone and the tube filter system variations may provide greater cross-sectional flow area, each of these variations requires that the polymer flow change direction to pass through the filter system. In the case of the filter system with tubes, the polymer flow generally undergoes two right angle turns to get through the filter system. Such abrupt turning may alter the relationship between long chain polymers. Further, in the case of the filter system tubes directed downstream, the polymer flow is further broken up by being directed along separate paths into the array of tubes, then being split up in each tube for passage through the holes in the tube wall, after which the flow passing out of adjacent tubes comes together and must change direction again. This process is generally reversed where the tubes are directed upstream. In either case, the polymer flow may be expected to be turbulent, subjecting the polymer material to possible breakdown. Also, while the tube filter system structure may allow the use of thinner walls in the tubular support members, compared to the thickness of a flat filter support with comparable cross-sectional flow area for the polymer material, the closeness of the filter tubes combined with the circuitous paths the polymer material must follow provides a greater back pressure in the polymer material. In the case of the cone filter system, the flow of the extrusion material is directed outwardly against the outer wall of the tubular member housing the filter system.
Another filter system variation has been proposed for providing strength to the filter support while allowing the wall of the filter support to be thinner than in the case of a flat support. The proposed shape is a portion of a sphere with the convex surface facing upstream, and with the filter screen lying against the convex surface. A particular arrangement of holes through the wall of the support has also been proposed, consisting of holes all of which are longitudinally oriented parallel to the longitudinal axis of the filter system and the tubular housing containing the filter system. Further, the holes are arranged in circles centered on the longitudinal axis of the support, with each circle of holes positioned to intersect the concave surface of the support at equal increments of displacement along the longitudinal axis of the support. It will be appreciated that the distance along the support surface between adjacent rows decreases for rows positioned away from the vertex of the partial sphere. In general, the density of holes through the partial spherical surface increases for increased transverse distance away from the principle axis of the partial sphere. If all of the holes through the partial sphere support are of the same cross-sectional area, and if all of the holes in one circle are as mutually displaced along the surface as all of the holes in any other circle, then the cross-sectional area available for polymer flow through the filter system also increases for increased transverse distance away from the center of the tubular housing containing the filter system. Thus, the flow along the walls of the extruder apparatus at the filter system would tend to be faster than the flow along the center. Consequently, the extruder material may tend to move radially outwardly as the material approaches the filter system. Further, the length of the passage through the holes in the support increases with radial displacement of the holes from the central axis of the filter support, thus causing greater resistance to flow through the holes of longer passage and an increase in back pressure in the polymer material.
It would be desirable and advantageous to provide a filter system, or screen packing, whose support structure, or filter backing, can not only withstand the forces of the polymer flow being driven by the extruder apparatus, but minimizes potential damage to the polymers, minimizes back pressure in the extrusion material, minimizes resistance to the flow of the polymer material through the filter support and promotes laminar flow of the polymer material. The present invention provides a filter support which meets these objectives, and improves on the aforementioned proposed partial sphere with the aforementioned proposed arrangement of holes through the filter support.